Corrosion-resistant glass melt electrodes and methods of using them

ABSTRACT

In various embodiments, refractory-metal glass melt electrodes are single-crystalline, at least within an outer layer thereof.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/058,259, filed Oct. 1, 2014, the entiredisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to electrodesutilized for glass melting, in particular to such electrodes exhibitingenhanced corrosion resistance.

BACKGROUND

Glass is typically processed by batch heating and refining within amelting furnace. Glass batches are typically heated via burners, whichserve as the primary heat source, and from glass melt electrodesembedded in the wall of the melting furnace. The number of electrodesdepends upon the size of the melting furnace and the characteristics ofthe glass being processed. These glass melt electrodes introduceadditional thermal energy into the furnace by passing a current throughthe glass melt. Due to the extremely high temperatures required forglass melting (typically between approximately 1100° C. andapproximately 1700° C.), glass melt electrodes are typically formed ofrefractory metals, such as molybdenum (Mo) or tungsten (W), or alloysthereof. However, molten glass is often extremely corrosive andoxidative, and even such refractory metals may corrode and degrade afterlong periods of use.

Conventional electrodes are polycrystalline and typically have grainsizes no greater than 100 microns. One primary corrosion mode inconventional electrodes is corrosive attack on the refractory metalalong the crystalline grain boundaries thereof. Due to the increasedinteratomic disorder and free volume at grain boundaries, corrosiveelements (particularly polyvalent elements such as Pb, As, Sb, Co, Ni,or Mn) from the glass melt diffuse more quickly into the melt electrodeat the grain boundaries, leading to increased corrosion. This effect maybe mitigated somewhat by the incorporation of alloying elements (e.g.,Si, B, Pd, Pt, Ir, or Ru) within the electrode material, as the alloyingelements tend to preferentially accumulate at the grain boundaries andretard grain-boundary diffusion and related corrosive attack. However,such alloying elements may be quite expensive and may also diminish themechanical processability of the electrode material, making electrodeformation more time-consuming and expensive. In some cases, the alloyingelements may deleteriously contaminate the molten glass itself. Suchconsiderations limit the amount of alloying elements that may be addedto the electrode material, and thus grain boundary diffusion (andconcomitant corrosion) typically still occurs in conventionalelectrodes.

In view of the foregoing, there is a need for improved glass meltelectrodes that resist corrosion but remain amenable to mechanicalprocessing.

SUMMARY

In accordance with various embodiments of the present invention, glassmelt electrodes are composed of a single-crystal refractory metal (suchas Mo), at least in their outer surface layers. While the entireelectrode may be single crystalline, in some embodiments, thesingle-crystal outer surface layer surrounds a polycrystalline corecontaining a network of grain boundaries that does not penetrate into orthrough the outer surface layer. The grain boundaries present withinsuch a polycrystalline core may provide the electrode with increasedresistance to creep, particularly at the extreme temperatures utilizedin the glass melt furnace.

Glass melt electrodes in accordance with embodiments of the presentinvention may include, consist essentially, or consist of one or morerefractory metals such as Mo or W. In various embodiments of theinvention, the glass melt electrodes are substantially free of (i.e.,free of except for any present as unintentional impurities) of alloyingelements that tend to accumulate at grain boundaries (e.g., Si, B, Pd,Pt, Ir, and/or Ru). The glass melt electrodes may also be substantiallyfree of elements such as calcium and/or magnesium. In some embodiments,the glass melt electrodes are coated with a corrosion-resistant coating.For example, the coating may retard bulk oxidation of the electrodematerial. The coating may include, consist essentially of, or consistof, for example, MoSiB, MoZrO₂, or SiBC. Typically, the elements of thecoating remain at the surface of the electrode but do not appreciablydiffuse into the bulk of the electrode. Beneficially, the single-crystalouter surface layer of electrodes in accordance with embodiments of theinvention retards or substantially prevents attack on and/or detachmentof the coating. In conventional coated electrodes, corrosive speciesfrom the melt may penetrate through small gaps or microcracks in thecoating and diffuse along grain boundaries at the electrode/coatinginterface, resulting in corrosion and further loss of integrity of thecoating. In electrodes in accordance with embodiments of the presentinvention, no such grain boundaries are present on the outer surfacelayer, thereby maximizing the useful lifetime of the corrosion-resistantcoating and the electrode itself.

In additional embodiments of the present invention, at least the outersurface layer of the glass melt electrode is composed of a plurality oflarge grains, i.e., grains each having a grain size of at least 1 mm, atleast 5 mm, or even at least 10 mm. The large grain size vastly reducesthe grain-boundary density in at least the outer surface layer of theelectrodes, thereby substantially reducing any corrosion therealong. Asdescribed herein for single-crystal electrodes, the large-grainelectrodes may be coated with a corrosion-resistant coating.

In an aspect, embodiments of the invention feature a glass meltelectrode that includes, consists essentially of, or consists of a basesized and shaped for engagement with an interior surface of aglass-melting furnace, and an elongated shaft extending from the base.At least an outer surface layer of the shaft is single-crystalline. Atleast the outer surface layer of the shaft includes, consistsessentially of, or consists of one or more refractory metals.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least an outer surface layer of thebase may be single-crystalline. The outer surface layer of the base maybe disposed around a non-single-crystalline (e.g., polycrystalline oramorphous) core. The entire volume of the base may besingle-crystalline. The base may include, consist essentially of, orconsist of one or more refractory metals that are the same as ordifferent than the one or more refractory metals of the outer surfacelayer of the shaft. The base and the shaft may be portions of a seamlessunitary structure. The outer surface layer (or even the entire volume)of the shaft and/or the base may be substantially free of grainboundaries. The one or more refractory metals may include, consistessentially of, or consist of Mo and/or W. The one or more refractorymetals may include, consist essentially of, or consist of Nb, Ta, and/orRe. The one or more refractory metals may include, consist essentiallyof, or consist of Ti, V, Cr, Zr, Hf, Ru, Rh, Os, and/or Ir.Substantially all of the shaft (i.e., substantially the entire volume ofthe shaft) may be single-crystalline. The outer surface layer of theshaft may be disposed around a core that is not single-crystalline. Thecore may be, e.g., polycrystalline or amorphous. The core may include,consist essentially of, or consist of one or more of the one or morerefractory metals. The core may include, consist essentially of, orconsist of one or more refractory metals different from the one or morerefractory metals of the outer surface layer. A thickness of the outersurface layer may be between approximately 100 μm and approximately 1500μm. A corrosion-resistant coating may be disposed on at least a portionof the shaft (e.g., on substantially the entire outer surface layer ofthe shaft) and/or at least a portion of the base (e.g., on substantiallythe entire outer surface layer of the base). The corrosion-resistantcoating may include, consist essentially of, or consist of MoSiB,MoZrO₂, and/or SiBC. A thickness of the corrosion-resistant coating maybe between approximately 100 μm and approximately 500 μm. The shaft maydefine a hollow cooling channel therewithin.

In another aspect, embodiments of the invention feature a method ofprocessing glass. A glass-melting furnace is provided. One or more glassmelt electrodes protrude from an inner wall of the glass-melting furnaceinto an inner volume of the glass-melting furnace. Glass material isdisposed within the inner volume of a glass-melting furnace. The glassmaterial is melted at least in part by applying electrical current tothe one or more glass melt electrodes. At least one glass melt electrodeis at least partially immersed in the melted glass material. At least anouter surface layer of at least one of the glass melt electrodes issingle-crystalline and includes, consists essentially of, or consists ofone or more refractory metals. The one or more glass melt electrodesresist corrosion during the melting of the glass material.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The one or more refractory metals mayinclude, consist essentially of, or consist of Mo and/or W. The one ormore refractory metals may include, consist essentially of, or consistof Nb, Ta, and/or Re. The one or more refractory metals may include,consist essentially of, or consist of Ti, V, Cr, Zr, Hf, Ru, Rh, Os,and/or Ir. Substantially an entire volume of at least one glass meltelectrode may be single-crystalline. The outer surface layer of at leastone glass melt electrode may be disposed around a non-single-crystallinecore. The core may be, e.g., polycrystalline or amorphous. The core mayinclude, consist essentially of, or consist of one or more of the one ormore refractory metals. The core may include, consist essentially of, orconsist of one or more refractory metals different from the one or morerefractory metals of the outer surface layer. A thickness of the outersurface layer may be between approximately 100 μm and approximately 1500μm. A corrosion-resistant coating may be disposed on at least a portionof the shaft (e.g., on substantially the entire outer surface layer ofthe shaft) and/or at least a portion of the base (e.g., on substantiallythe entire outer surface layer of the base) of at least one glass meltelectrode. The corrosion-resistant coating may include, consistessentially of, or consist of MoSiB, MoZrO₂, and/or SiBC. A thickness ofthe corrosion-resistant coating may be between approximately 100 μm andapproximately 500 μm. At least one glass melt electrode may define ahollow cooling channel therewithin.

In yet another aspect, embodiments of the invention feature a glass meltelectrode that includes, consists essentially of, or consists of a basesized and shaped for engagement with an interior surface of aglass-melting furnace, and an elongated shaft extending from the base.At least an outer surface layer of the shaft is composed of largecrystalline grains each having a grain size larger than approximately 1mm. At least the outer surface layer of the shaft includes, consistsessentially of, or consists of one or more refractory metals.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least an outer surface layer of thebase may be composed of the large crystalline grains. The outer surfacelayer of the base may be disposed around a non-single-crystalline (e.g.,polycrystalline or amorphous) core. The entire volume of the base may becomposed of the large crystalline grains. The base may include, consistessentially of, or consist of one or more refractory metals that are thesame as or different than the one or more refractory metals of the outersurface layer of the shaft. The base and the shaft may be portions of aseamless unitary structure. The one or more refractory metals mayinclude, consist essentially of, or consist of Mo and/or W. The one ormore refractory metals may include, consist essentially of, or consistof Nb, Ta, and/or Re. The one or more refractory metals may include,consist essentially of, or consist of Ti, V, Cr, Zr, Hf, Ru, Rh, Os,and/or Ir. Substantially all of the shaft (i.e., substantially theentire volume of the shaft) may be composed of the large crystallinegrains. The outer surface layer of the shaft may be disposed around acore that is not composed of the large crystalline grains. The core maybe, e.g., amorphous or polycrystalline with an average grain sizesmaller than the grain size of the large crystalline grains. The coremay include, consist essentially of, or consist of one or more of theone or more refractory metals. The core may include, consist essentiallyof, or consist of one or more refractory metals different from the oneor more refractory metals of the outer surface layer. A thickness of theouter surface layer may be between approximately 100 μm andapproximately 1500 μm. A corrosion-resistant coating may be disposed onat least a portion of the shaft (e.g., on substantially the entire outersurface layer of the shaft) and/or at least a portion of the base (e.g.,on substantially the entire outer surface layer of the base). Thecorrosion-resistant coating may include, consist essentially of, orconsist of MoSiB, MoZrO₂, and/or SiBC. A thickness of thecorrosion-resistant coating may be between approximately 100 μm andapproximately 500 μm. The shaft may define a hollow cooling channeltherewithin. The grain size of each of the large crystalline grains maybe larger than approximately 5 mm, or even larger than approximately 10mm.

In another aspect, embodiments of the invention feature a method ofprocessing glass. A glass-melting furnace is provided. One or more glassmelt electrodes protrude from an inner wall of the glass-melting furnaceinto an inner volume of the glass-melting furnace. Glass material isdisposed within the inner volume of a glass-melting furnace. The glassmaterial is melted at least in part by applying electrical current tothe one or more glass melt electrodes. At least one glass melt electrodeis at least partially immersed in the melted glass material. At least anouter surface layer of at least one of the glass melt electrodes (i) iscomposed of large crystalline grains each having a grain size largerthan 1 mm, and (ii) includes, consists essentially of, or consists ofone or more refractory metals. The one or more glass melt electrodesresist corrosion during the melting of the glass material.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The one or more refractory metals mayinclude, consist essentially of, or consist of Mo and/or W. The one ormore refractory metals may include, consist essentially of, or consistof Nb, Ta, and/or Re. The one or more refractory metals may include,consist essentially of, or consist of Ti, V, Cr, Zr, Hf, Ru, Rh, Os,and/or Ir. Substantially an entire volume of at least one glass meltelectrode may be composed of the large crystalline grains. The outersurface layer of at least one glass melt electrode may be disposedaround a non-single-crystalline core. The core may be, e.g., amorphousor polycrystalline with an average grain size smaller than the grainsize of the large crystalline grains. The core may include, consistessentially of, or consist of one or more of the one or more refractorymetals. The core may include, consist essentially of, or consist of oneor more refractory metals different from the one or more refractorymetals of the outer surface layer. A thickness of the outer surfacelayer may be between approximately 100 μm and approximately 1500 μm. Acorrosion-resistant coating may be disposed on at least a portion of theshaft (e.g., on substantially the entire outer surface layer of theshaft) and/or at least a portion of the base (e.g., on substantially theentire outer surface layer of the base) of at least one glass meltelectrode. The corrosion-resistant coating may include, consistessentially of, or consist of MoSiB, MoZrO₂, and/or SiBC. A thickness ofthe corrosion-resistant coating may be between approximately 100 μm andapproximately 500 μm. At least one glass melt electrode may define ahollow cooling channel therewithin. The grain size of each of the largecrystalline grains may be larger than approximately 5 mm, or even largerthan approximately 10 mm.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean ±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be detectable via chemical analysis but do notcontribute to function. As used herein, “consisting essentially of atleast one metal” refers to a metal or a mixture of two or more metalsbut not compounds between a metal and a non-metallic element or chemicalspecies such as oxygen, silicon, or nitrogen (e.g., metal nitrides,metal silicides, or metal oxides); such non-metallic elements orchemical species may be present, collectively or individually, in traceamounts, e.g., as impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic cross-section of a glass melt electrode inaccordance with various embodiments of the invention;

FIG. 2 is a schematic cross-section of a glass melt electrode inaccordance with various embodiments of the invention;

FIG. 3 is a micrograph depicting a portion of a polycrystalline glassmelt electrode after a corrosion test;

FIGS. 4A and 4B are micrographs depicting portions of a coatedpolycrystalline glass melt electrode after a corrosion test;

FIG. 5 is a micrograph depicting a portion of a coatedsingle-crystalline glass melt electrode in accordance with variousembodiments of the invention after a corrosion test; and

FIG. 6 is a graph comparing results of two corrosion tests on a varietyof polycrystalline glass melt electrodes and a single-crystal glass meltelectrode in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts a glass melt electrode 100 in accordance with variousembodiments of the present invention. As shown, glass melt electrode 100includes a base 110 that is sized and shaped for attachment within aglass-melting furnace (e.g., engagement on an interior wall or bottomsurface thereof), where it electrically connects to a power source. Inexemplary embodiments, the base 110 is threaded and/or tapered, and/orhas a diameter (or lateral dimension such as a width) different (e.g.,smaller) than the diameter (or lateral dimension such as a width) of theshaft of the electrode. Extending from the base is a shaft 120, at leasta portion of which may be substantially cylindrical. The end 125 of theshaft 120 opposite the base may be substantially flat, as shown, or maybe rounded. In some embodiments, the end 125 is roughly teardrop-shaped,as detailed in U.S. Pat. No. 8,743,926, filed on Aug. 10, 2010 (the '926patent), the entire disclosure of which is incorporated by referenceherein. The electrode 110 may be a single, unified part or may includeor consist essentially of multiple parts that fit together. For example,multiple sections may be threaded to engage with and attach to eachother. For example, the base 110 and/or a section of the shaft 125including the end 125 may be a discrete piece attachable and detachablefrom the remaining portion(s) of the electrode 100. In variousembodiments of the invention, the shaft 120 (and, in some embodiments,the base 110) includes, consists essentially of, or consists of at leastone single-crystalline refractory metal, e.g., Nb, Ta, Re, Mo, and/or W.In various embodiments of the invention, the shaft 120 (and, in someembodiments, the base 110) includes, consists essentially of, orconsists of at least one high-melting-temperature metal (e.g., a metalhaving a melting point higher than approximately 1850° C.), e.g., Ti, V,Cr, Zr, Hf, Ru, Rh, Os, and/or Ir. In various embodiments of theinvention, the shaft 120 (and, in some embodiments, the base 110)includes, consists essentially of, or consists of an alloy or mixture oftwo or more refractory metals and/or high-melting-temperature metalslisted above. The lack of grain boundaries within the shaft 120beneficially retards or substantially eliminates corrosion due to attackby corrosive elements within the glass melt in which the shaft 120 isimmersed during use.

The shaft 120 of the electrode 100 may incorporate a cooling channel 130therewithin, as described in the '926 patent. Water or another coolant(e.g., a liquid or a gas) may be flowed through the cooling channel 130to maintain the electrode 100 at a lower temperature during theglass-melting operation. In addition, all or a portion of the electrode100 (including, in some embodiments, the base 110) may have acorrosion-resistant coating 140 thereon. The coating 140 may retard orsubstantially eliminate bulk oxidation and/or corrosion of the electrode100 via interaction between corrosive elements in the bath of moltenglass and the material of the shaft 120. The coating 140 may include,consist essentially of, or consist of, for example, MoSiB, MoZrO₂, orSiBC. The coating 140 may have a thickness between, for example,approximately 100 μm and approximately 500 μm. The coating may beapplied to all or part of the electrode 100 by, e.g., sputtering, plasmaspray, cold spray, and/or chemical-vapor deposition.

FIG. 2 depicts a glass melt electrode 200 in accordance with variousembodiments of the present invention. Glass melt electrode 200 issimilar to glass melt electrode 100 but is not composed entirely ofsingle-crystalline material. Instead, electrode 200 features an outersurface layer 210 that is single-crystalline and that surrounds apolycrystalline (or, in some embodiments, substantially amorphous) core220. The thickness of the outer surface layer 210 is generallysufficiently thick to retard or sufficiently prevent the diffusion ofcorrosive elements from the glass melt into the core 220, which is moresusceptible to corrosion due to the presence of grain boundaries (or agenerally more disordered interatomic structure) therein. For example,the outer surface layer 210 may have a thickness between approximately100 μm and approximately 1500 μm. The polycrystalline core 220 mayprovide the electrode 200 with enhanced resistance to creep deformationdue to the network of grain boundaries (or other atomic-level disorder)therewithin, particularly at the high temperatures within theglass-melting furnace during operation. Like the shaft 120 of electrode100, the outer surface layer 210 of electrode 200 may include, consistessentially of, or consist of a single-crystalline refractory metal,e.g., Nb, Ta, Re, Mo, and/or W. In various embodiments of the invention,the surface layer 210 includes, consists essentially of, or consists ofat least one high-melting-temperature metal, e.g., Ti, V, Cr, Zr, Hf,Ru, Rh, Os, and/or Ir. The core 220 may include, consist essentially of,or consist of a polycrystalline and/or amorphous material (e.g., one ormore metals), preferably (but not necessarily) the same material ofwhich the outer surface layer 210 is composed. For example, the core 220may include, consist essentially of, or consist of Nb, Ta, Re, Mo, W,Ti, V, Cr, Zr, Hf, Ru, Rh, Os, and/or Ir. The electrode 200 may alsoincorporate a cooling channel 130 (not shown in FIG. 2 for clarity) asdescribed above and illustrated in FIG. 1.

In additional embodiments of the present invention, electrode 100 orouter surface layer 210 is composed of a plurality of large grains,i.e., grains each having a grain size of at least 1 mm, at least 5 mm,or even at least 10 mm. The large grain size vastly reduces thegrain-boundary density in at least the outer surface layer of theelectrodes, thereby substantially reducing any corrosion therealong. Inembodiments in which outer surface layer 210 of electrode 200 iscomposed of large grains, the core 220 may include, consist essentiallyof, or consist of amorphous material or polycrystalline material havinga grain size smaller (e.g., at least 10 times smaller) than that ofsurface layer 210.

The glass melt electrodes in accordance with embodiments of the presentinvention may be advantageously utilized to melt glass (for example, ina glass-melting furnace) while resisting corrosion from the bath ofmolten glass in which the electrodes are partially or completelyimmersed. The electrodes may be utilized with any of a variety ofdifferent types of glass, e.g., soda-lime glass and/or borosilicateglass.

Glass melt electrodes in accordance with embodiments of the presentinvention may be fabricated by any of a variety of different techniques.In some embodiments, the electrode is initially fabricated as apolycrystalline structure via, e.g., powder metallurgy techniques (e.g.,pressing and sintering, hot-isostatic pressing, cold-isostatic pressing,etc.) or casting, and then processed such that the outer surface layer(and, in some embodiments, at least the entire shaft) of the electrodeis single crystalline or composed of few large grains. For example,techniques such as zone melting and/or static or dynamic abnormalgrain-growth techniques (utilizing, e.g., cyclic annealing treatments)may be utilized to promote formation of a few grains or a single grainin at least the outer surface layer, thereby reducing or substantiallyeliminating grain boundaries therewithin. In other embodiments, theelectrode may be initially fabricated as a single-crystalline bodyutilizing techniques such as investment casting.

For example, electrodes in accordance with embodiments of the inventionmay be fabricated utilizing a process that includes a floating-zoneprocess (i.e., zone melting). In such a process, a polycrystalline oramorphous Mo rod (fabricated by, e.g., powder metallurgy techniques orcasting) is disposed within a vacuum furnace. A single crystal seed(which may include, consist essentially of, or consist of, e.g., Mo) isattached to one end of the Mo rod, and then a small portion of the rodproximate the seed is heated using, for example, induction heating orradiation heating (e.g., using an induction coil or a resistanceheater). The furnace is evacuated or purged with an inert gas. Theapplied heat forms a narrow molten zone, and then the heating apparatusis moved (or, equivalently, the rod is translated relative to theheating apparatus) along the length of the rod. Within the molten zone,single-crystal Mo nucleates from the seed, and the relative movement ofthe heating apparatus and the rod causes the single-crystal Mo region togrow along the length of the Mo rod while “consuming” thepolycrystalline or amorphous portion of the rod. In various embodiments,the interior portion of the rod may be maintained polycrystalline oramorphous by, e.g., melting only the outer portion of the Mo rod and/orutilizing a Mo seed that has a polycrystalline or amorphous centerregion surrounded by a single-crystalline outer region.

Electrodes in accordance with embodiments of the present invention mayalso be fabricated via “needle-eye” zone melting that enablesfabrication of electrodes having a larger diameter than the heatingcoil. Like the zone melting process detailed above, a heating coil istranslated relative to an electrode to locally melt and reform theelectrode as a single crystal. In the needle-eye process, most of theinitial polycrystalline or amorphous rod has a diameter larger than theopening in the heating coil, but one end of the rod tapers down to adiameter smaller than the opening. The single-crystal seed is affixed tothis tapered end of the rod, and the heating coil slides over the seededend. As the heating coil is translated relative to the rod (i.e., withthe coil and/or rod actually being moved), the molten zone within theheating coil has the smaller diameter (i.e., smaller than the coilopening), while the processed portion of the rod expands in diameterduring the re-solidification process and has a final diameter largerthan that of the molten zone (e.g., approximately equal to the largerdiameter of the unprocessed rod). Exemplary electrodes fabricated bythis process may have diameters up to 3-4 inches, or even larger.

In other embodiments, electrodes in accordance with embodiments of thepresent invention may be fabricated utilizing, at least in part, staticor dynamic abnormal grain-growth techniques. In both techniques, apolycrystalline Mo rod produced by, e.g., powder metallurgy techniques(e.g., pressing and sintering Mo powder) or arc casting, is heated to ahigh temperature to trigger the abnormal grain growth of a single grainor of a few grains. In various embodiments, before the high-temperaturetreatment, the polycrystalline Mo rod may be mechanically deformed by,for example, a minimum of 75% RA (reduction in cross-sectional area).During the heat treatment, one or a few grains expand in volume at theexpense of the other grains in the rod (which are consumed or vastlyreduced in size) until substantially all (or a substantial portion) ofthe rod is single-crystalline or composed of a few large grains. Indynamic abnormal grain growth, the Mo rod is also placed under tensilestress to initiate the grain growth, while static abnormal grain growthdoes not utilize added stress.

In an exemplary static abnormal grain growth process, a Mo billet may beproduced by, for example, pressing and sintering of Mo powder. The Mopowder may also be blended with a small amount (e.g., approximately 10ppm to approximately 50 ppm) of an oxide powder (e.g., CaO or MgO).After sintering of the billet, the billet may be rolled or otherwiseworked at an elevated temperature (e.g., approximately 1000° C.) into arod. The rod may then be heated to an even higher temperature. Forexample, the nominal annealing temperature may be approximately 2000°C., or even higher. The rod may be heated for, e.g., approximately 1hour, or even longer. The abnormal grain growth will be triggered at apoint along the length of the rod, and the single grain or few grainswill grow along the length of the rod. If the entire rod does not end upas a single crystal (e.g., if one or both ends still contain multiplecrystalline grains), then any polycrystalline portions of the rod may becut away to produce a single-crystalline electrode in accordance withembodiments of the present invention.

In an exemplary dynamic abnormal grain growth process, a substantiallypure Mo rod may be produced by, for example, arc melting or powdermetallurgy techniques. In various embodiments, the polycrystalline Morod may be mechanically deformed by, for example, a minimum of 75% RA.The rod may be fixtured (i.e., attached at both ends) in an apparatus(e.g., a tensile-test apparatus) that puts the rod in a state of tensilestrain by pulling one or both ends. The rod is heated during the pullingto, for example, a temperature greater than approximately 1400° C.(e.g., between approximately 1400° C. and approximately 2200° C.). Therod is slowly pulled at, for example, a constant true-strain rate lessthan approximately 10⁻⁴ s⁻¹. During the heating and pulling, theabnormal grain growth will be triggered, signified by a fairly largeload drop detectable by the pulling apparatus. Once that load begins toagain increase, the dynamic abnormal grain growth process issubstantially complete. As with the static technique described above, ifthe entire rod does not end up as a single crystal, then anypolycrystalline portions of the rod may be cut away to produce asingle-crystalline electrode in accordance with embodiments of thepresent invention.

EXAMPLE 1

A corrosion test was performed on three different electrodes. Electrode#1 was a conventional polycrystalline Mo glass melt electrode, Electrode#2 was a conventional polycrystalline Mo glass melt electrode with acorrosion-resistant coating (MoSiB) thereon, and Electrode #3 was asingle-crystalline Mo glass melt electrode with a corrosion-resistantcoating (MoSiB) thereon in accordance with embodiments of the presentinvention. Each of the electrodes was cylindrical and had a length of5.25 inches and a diameter of 0.625 inches. The electrodes were immersedin molten soda-lime glass (SiO₂—Na₂O—CaO—MgO—Al₂O₃—Fe₂O₃—K₂O) at 1100°C., with alternating current flowing through the electrodes, for aperiod of 100 hours. Electrodes #1 and #2 had grain sizes ranging fromapproximately 30 microns to approximately 100 microns.

FIG. 3 is a scanning electron microscopy (SEM) micrograph showing theresults for Electrode #1. The micrograph depicts a region near thesurface of the electrode (designated as electrode 300). As shown, themolten glass penetrated into the electrode 300 via grain boundaries 310,forming large glass regions 320 within the bulk of the electrode 300 andresulting in the detachment of fairly large portions of the electrode300. In addition, an oxidized layer 330 including or consistingessentially of MoO₂ was formed between the glass-containing region 320and the inner portion of the Mo electrode 300 due to bulk oxidationenabled by the lack of any corrosion-resistant coating on the surface ofthe electrode.

FIG. 4A is an SEM micrograph showing the results for Electrode #2. Dueto the presence of the corrosion-resistant coating, little or nooxidized layer is formed on Electrode #2 (designated as electrode 400having corrosion-resistant coating 410 thereon). However, corrosiveelements from the glass melt have penetrated through the coating 410 anddiffused along grain boundaries 420 of the electrode 400, resulting incorrosion within the bulk of the electrode and glass-containing regions430. This grain-boundary corrosion can lead to delamination and loss ofthe coating, as shown in the SEM micrograph of FIG. 4B.

FIG. 5 is an SEM micrograph showing the results for Electrode #3(designated as electrode 500 having corrosion-resistant coating 510thereon). As shown, the corrosion-resistant coating 510 has beensubjected to attack by the molten glass, but no corrosive elements fromthe glass have penetrated through the coating 510 and into thesingle-crystalline surface layer of the electrode 500, thus improvingthe lifetime of the coating 510 and of the electrode 500 itself.

EXAMPLE 2

Two corrosion tests were performed on several different electrodes. Eachof the electrodes was cylindrical and had a length of 5.25 inches and adiameter of 0.625 inches. In the first test, the electrodes wereimmersed in molten soda-lime glass (SiO₂—Na₂O—CaO—MgO—Al₂O₃—Fe₂O₃—K₂O)at 1200° C. for a period of 12 days. In the second test, the electrodeswere immersed in molten borosilicate glass(SiO₂—CaO—Al₂O₃—MgO—B₂O₃—Na₂O—F—TiO₂—Fe₂O₃) at 1300° C. for a period of12 days. After each test, the depth within each electrode of maximumpenetration of glass constituents was measured. FIG. 6 is a graph of theresults of the tests for, from left to right, (1) polycrystalline Moelectrodes fabricated by powder metallurgy (e.g., pressing and sinteringof Mo powder), (2) polycrystalline arc-cast Mo electrodes, (3)polycrystalline powder-metallurgy electrodes composed of a TZM alloy(i.e., a titanium (Ti)-zirconium (Zr)-Mo alloy, in this caseapproximately 0.5% Ti, 0.1% Zr, and the balance Mo), (4) polycrystallinearc-cast electrodes composed of the TZM alloy, (5) polycrystallineelectrodes composed of an alloy of Mo with 30% W, (6) polycrystallineelectrodes composed of oxide-dispersion-strengthened (ODS) Mo-lanthanum(La) alloy (i.e., Mo containing a dispersion of lanthanum oxideparticles (present at, e.g., 0.3%-1.1% by weight)), and (7)single-crystal Mo in accordance with embodiments of the presentinvention. As shown in FIG. 6, the single-crystal Mo electrodesexhibited no detectable penetration of the components of the glassmelts, while all of the other samples, which had grain sizes rangingfrom approximately 30 microns to approximately 100 microns, exhibitedbetween 100 μm and 600 μm of penetration. These results emphasize thesuperior performance and lifetime of single-crystalline electrodes inaccordance with embodiments of the invention, due at least in part tothe absence of grain boundaries at least at exposed surfaces thereof.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A glass melt electrode comprising: a base sizedand shaped for engagement with an interior surface of a glass-meltingfurnace; and an elongated shaft extending from the base, wherein (i) atleast an outer surface layer of the shaft is single-crystalline andcomprises one or more refractory metals, (ii) the outer surface layer ofthe shaft is disposed around a polycrystalline core, and (iii) athickness of the outer surface layer is between approximately 100 μm andapproximately 1500 μm.
 2. The glass melt electrode comprising: a basesized and shaped for engagement with an interior surface of aglass-melting furnace; an elongated shaft extending from the base; and acorrosion-resistant coating disposed on at least a portion of the shaft,wherein the corrosion-resistant coating comprises at least one of MoSiB,MoZrO₂, or SiBC.
 3. The glass melt electrode of claim 1, wherein the oneor more refractory metals comprise at least one of Mo or W.
 4. The glassmelt electrode of claim 1, wherein the one or more refractory metalscomprise at least one of Nb, Ta, or Re.
 5. The glass melt electrode ofclaim 1, wherein the one or more refractory metals comprise at least oneof Ti, V, Cr, Zr, Hf, Ru, Rh, Os, or Ir.
 6. The glass melt electrode ofclaim 1, wherein the core comprises the one or more refractory metals.7. The glass melt electrode of claim 1, further comprising acorrosion-resistant coating disposed on at least a portion of the shaft.8. The glass melt electrode of claim 7, wherein the corrosion-resistantcoating comprises at least one of MoSiB, MoZrO₂, or SiBC.
 9. The glassmelt electrode of clam 7, wherein a thickness of the corrosion-resistantcoating is between approximately 100 μm and approximately 500 μm. 10.The glass melt electrode of claim 1, wherein the shaft defines a hollowcooling channel therewithin.
 11. The glass melt electrode of claim 2,wherein the one or more refractory metals comprise at least one of Mo orW.
 12. The glass melt electrode of claim 2, wherein the one or morerefractory metals comprise at least one of Nb, Ta, or Re.
 13. The glassmelt electrode of claim 2, wherein the one or more refractory metalscomprise at least one of Ti, V, Cr, Zr, Hf, Ru, Rh, Os, or Ir.
 14. Theglass melt electrode of claim 2, wherein substantially all of the shaftis single crystalline.
 15. The glass melt electrode of claim 2, whereinan outer surface layer of the shaft is single-crystalline and disposedaround a polycrystalline core.
 16. The glass melt electrode of claim 15,wherein the core comprises the one or more refractory metals.
 17. Theglass melt electrode of claim 2, wherein a thickness of thecorrosion-resistant coating is between approximately 100 μm andapproximately 500 μm.
 18. The glass melt electrode of claim 2, whereinthe shaft defines a hollow cooling channel therewithin.
 19. The glassmelt electrode of claim 1, wherein (i) the shaft extends from the baseto an end, and (ii) a diameter of the shaft, from the base to the end,is substantially constant.
 20. The glass melt electrode of claim 1,wherein the shaft extends from the base to a flat end.
 21. The glassmelt electrode of claim 2, wherein (i) the shaft extends from the baseto an end, and (ii) a diameter of the shaft, from the base to the end,is substantially constant.
 22. The glass melt electrode of claim 2,wherein the shaft extends from the base to a flat end.